Inverters can convert electrical power of an energy generation unit from direct current to alternating current and feed it into an energy supply grid, for example an AC voltage grid, and/or supply it directly to an electrical load for the operation thereof. The energy generation unit used in this case can be, in particular, generators, which generate electrical power from renewable sources, for example photovoltaic generators or wind generators.
For the connection of an inverter to an AC voltage grid, it is necessary, for normative reasons, to make it possible to disconnect the energy generation unit from the AC voltage grid with galvanic isolation and single-fault safety. Disconnection of this kind can be ensured, for example, by way of a grid disconnection point disclosed in EP 2 228 895 A1, which has two relays connected in series.
EP 1 857 825 A1 discloses a measurement arrangement comprising a grounding point for identifying the insulation resistance of a live electrical apparatus or an installation comprising a supply voltage comprising a positive pole and a negative pole. In this case, two switches or one corresponding changeover switch are provided, which each establish a current path between one of the two poles and a grounding point in order to identify the total insulation resistance resulting in the event of the occurrence of one or more insulation faults with any desired potential reference. In order to identify the insulation resistance, two successive measurements are performed, wherein, in the first of these measurements, the first switch is closed while the second switch is open, and wherein, in the second of these measurements, the first switch is open while the second switch is closed. During the measurements, the current flowing via the respectively established connection to the grounding point is measured. The insulation resistance is calculated from the two measured currents, in respect of which the cited document provides a detailed instruction.
WO 2013/178654 A1 discloses an inverter comprising at least one half-bridge having two active switching elements for driving an output current and comprising an apparatus for measuring an insulation resistance, wherein the apparatus successively establishes a connection of the two ungrounded poles of a link circuit voltage of the inverter to a grounding point and measures a current flowing to the grounding point via the connection. The apparatus has a ground switch, which can be used to connect a center point of the half-bridge, which center point is located between the switching elements, to the grounding point. The apparatus connects the center point, which is connected to the grounding point, by means of the switching elements of the half-bridge successively to the two poles of the link circuit voltage of the inverter applied to the half-bridge, in order to establish the connection for measuring the current flowing to the grounding point.
DE 10 2011 018 229 A1 discloses a circuit arrangement for the potential isolation of an inverter of an energy generation installation from a grid, which circuit arrangement comprises a series circuit composed of two switching elements, wherein a measurement device for detecting a potential or a potential shift at the center contact is provided between the two switching elements of the series circuit.
DE 10 2013 104 629 A1 discloses an energy generation device comprising a converter and a disconnection device for the potential isolation of the converter from a grid, wherein the disconnection device comprises a series circuit composed of two switch elements. The energy generation device can comprise an insulation measurement device in order to recognize a fault in one of the switch elements based on an insulation resistance determined by the insulation measurement device while the respective other of the switch elements is actuated to close.
DE 10 2011 122 359 A1 discloses an inverter, the output-side phase conductor of which is guided in each case via a series circuit composed of two switching contacts, wherein the values of the voltages on the phase conductors can be detected by means of measurement apparatuses, and a method for checking the function of electromechanical switches.
The known measurement arrangements can be specifically provided for monitoring a photovoltaic installation which feeds electrical energy from a photovoltaic generator into an AC grid for an insulation fault. The components to be provided in an inverter for forming the known measurement arrangements are numerous and increase the overall costs of the inverter considerably.